Laboratory Measurements of X-Ray Emission from Highly Charged Argon Ions

TL;DR

This study uses laboratory measurements with an electron beam ion trap to test if Argon ion emissions could explain the mysterious 3.5 keV X-ray line in galaxy clusters, finding they cannot account for it.

Contribution

The paper provides experimental validation of atomic models for Argon ions, specifically testing their role in the 3.5 keV line observed in galaxy clusters.

Findings

Argon He-beta DR flux is too weak to explain the 3.5 keV line.

Experimental results rule out Argon He-beta DR as a significant source of the 3.5 keV emission.

Comparison between atomic theory and laboratory measurements enhances model accuracy.

Abstract

Uncertainties in atomic models will introduce noticeable additional systematics in calculating the flux of weak dielectronic recombination (DR) satellite lines, affecting the detection and flux measurements of other weak spectral lines. One important example is the Ar XVII He-beta DR, which is expected to be present in emission from the hot intracluster medium (ICM) of galaxy clusters and could impact measurements of the flux of the 3.5 keV line that has been suggested as a secondary emission from a dark matter interaction. We perform a set of experiments using the Lawrence Livermore National Laboratory's electron beam ion trap (EBIT-I) and the X-Ray Spectrometer quantum calorimeter (XRS/EBIT), to test the Ar XVII He-beta DR origin of the 3.5 keV line. We measured the X-ray emission following resonant DR onto helium-like and lithium-like Argon using EBIT-I's Maxwellian simulator mode at a simulated electron temperature of Te=1.74 keV. The measured flux of the Ar XVII He-beta DR lined is too weak to account for the flux in the 3.5 keV line assuming reasonable plasma parameters. We, therefore, rule out Ar XVII He-beta DR as a significant contributor to the 3.5 keV line. A comprehensive comparison between the atomic theory and the EBIT experiment results is also provided.